Recognized as the most common inherited cause of intellectual disability and autism spectrum disorder.
Incidence is approximately 1 in 4000 to 5000 males and 1 in 8000 females.
Accounts for 30-50% of X-linked intellectual disability.
Genetics and Pathogenesis
Caused by a dynamic mutation involving the expansion of CGG trinucleotide repeats in the 5’ untranslated region of the FMR1 gene located on chromosome Xq27.3.
Exhibits anticipation, known as the Sherman paradox, where repeats expand in successive generations primarily through maternal transmission.
Inheritance follows an X-linked dominant pattern with reduced penetrance in females secondary to X-inactivation.
Hypermethylation of the expanded sequence causes transcriptional silencing and the subsequent absence of the fragile X mental retardation protein (FMRP).
FMRP regulates mRNA translation at synapses; its loss impairs synaptic plasticity and dendritic spine maturation, leading to cellular hyperexcitability.
CGG Repeat Classification
Allele Type
Repeat Number
Clinical Consequence
Normal
<45
Unaffected phenotype.
Intermediate
45-54
Unaffected, but may exhibit instability during meiosis.
Premutation
55-200
Associated with RNA toxicity leading to Fragile X-associated Tremor/Ataxia Syndrome (FXTAS) or Primary Ovarian Insufficiency (FXPOI).
Full Mutation
>200
Classic Fragile X syndrome resulting from absent FMRP.
Clinical Features
Patient Group
Characteristic Manifestations
Affected Males
Moderate to severe intellectual disability (IQ 40-55), language delay, and echolalia. Autism spectrum features (60-80%), hand flapping, and hyperactivity. Long narrow face, prominent forehead, large protruding ears, and macrocephaly. Hallmark post-pubertal macroorchidism (>30-50 mL). Seizures (15-20%) and mitral valve prolapse.
Affected Females
Milder phenotype seen in 50% due to skewed X-inactivation. Mild intellectual disability (IQ 70-85), emotional lability, anxiety, and premature ovarian failure.
Premutation Carriers
Females face a 20-25% risk of premature menopause before age 40 (FXPOI). Males over 50 years risk developing FXTAS, characterized by tremor, ataxia, parkinsonism, and cognitive decline.
Investigations
Molecular genetic testing of FMR1 CGG repeat size and methylation status represents the gold standard for diagnosis.
Triplet repeat primed PCR (TP-PCR) or capillary electrophoresis serves as the first-line test to detect all alleles including large expansions.
Southern blot with methylation analysis confirms full mutations, evaluates methylation status, and identifies mosaicism.
Standard PCR alone is inadequate as it fails to amplify large expansions.
Management Principles
Requires multidisciplinary care emphasizing early intervention, including speech, occupational, and physical therapy.
Behavioral therapy utilizing applied behavior analysis for autism features.
Pharmacotherapy includes stimulants for attention deficit hyperactivity disorder, selective serotonin reuptake inhibitors for anxiety, and valproate or levetiracetam for seizures. Carbamazepine should be avoided if possible.
Genetic Counselling
Affected males transmit the premutation to all daughters, confirming no male-to-male transmission.
Carrier females face a 50% chance of transmitting the mutation; the risk of expansion to a full mutation corresponds with maternal repeat size, exceeding 90% if >100 repeats are present.
Prenatal diagnosis is accessible via chorionic villus sampling or amniocentesis utilizing TP-PCR.
